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This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 764089
Biomass characterization and analytical pyrolysis
ABC Salt Summer School – Aston University, 12-14 August ‘19
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Content
1. Recap of biomass composition
2. Basic physicochemical biomass characterization techniques
3. “Advanced” and thermal decomposition based characterization
4. Analytical pyrolysis
• Working principle
• Analytical pyrolysis of biomass
• As a “micro-scale” reactor: 2 case-studies
• Lignin pyrolysis
• Zeolite-catalyzed pyrolysis
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Recap of biomass composition
What is biomass composed of ? 4 main groups of compounds
1. Water
2. Inorganic compounds (called ash, as the content is determined by ashing): K, Ca, Mg, P, S, N, Si,...
3. Extractives: non-structural compounds which can be leached using water or solvents
• Sugars and starch
• Lipids (oils and fats), waxes and resins
• Proteins and peptides
4. Cell wall: structural compounds (allow the plant to grow upright), consisting of three kinds of
polymers:
• Hemicellulose
• Cellulose
• Lignin
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Recap of biomass composition
Plant
Glucose
eenheid
Lignine
Hemicellulose
Cellulose
microfibril
Plantaardige
cellen
celwand
(10 – 20 n
m)
Plant cellsPlant
Cell wall
Hemicellulose
Lignin
Glucose unit
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Recap of biomass composition
Cellulose
• Linear polymer chain of D-glucose, formula: (C6H10O5)n• DP ~ 10000 (degree of polymerization)
• Multiple cellulose chains are laid out in parallel, stabilized through H-bonds imparts (predominantly) a
crystalline structure
• The crystallinity also explains the rather high thermal stability, as well as the difficulty by which cellulose is
enzymatically hydrolyzed
• Not digestible by mammals, except ruminants
• In woody biomass: up to 40 to 50 wt.% (dry biomass) is cellulose
• Examples of ‘pure’ cellulose: paper, cotton
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Recap of biomass composition
Hemicellulose
• Branched polymer of different hexoses and pentoses (i.e. C5 and C6 sugars like xylose, glucose, mannose,
galactose en glucuronic acid), formula: (C5H8O4)n• DP ~ 100 to 200 (degree of polymerization), much smaller molecule than cellulose
• Low DP and amorphous, leading to low thermal stability and easy to hydrolyze with dilute acids or bases
• In woody biomasses: 20 to 30 wt.% (dry basis) is hemicellulose
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Recap of biomass composition
Lignin
• Complex amorphous polymer, with aromatic
functionalities, hydrophobic in nature
• Formula: (C31H34O11)n, remark that lignin
contains far less oxygen than sugar polymers
like cellulose and hemicellulose
• Is composed of three subunits (i.e.
monolignols), and are bound by ether and C-
C covalent bonds
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Recap of biomass composition
• Lignin functions as a ‘cement’ in between the cellulose and hemicellulose plant fibers.
• Different bond types different energies of dissociation complex thermal degradation pattern
• Softwoods (e.g. pine, spruce): mainly. G-units, while hardwood (e.g. oak): G and S; grasses: G + H + S
• Softwoods 25 – 35 wt.% lignin, hardwood 18 – 25 wt.% lignin content (on dry basis)
p-coumaryl alcohol (H-unit)
conipheryl alcohol (G-unit)
sinapyl alcohol (S-unit)
Lignin monomers Lignin bond types
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How to characterize biomass
Basic techniques to characterize biomass prior to thermochemical conversion:
• Physical
• Density (true, apparent and bulk)
• Thermal (conductivity and specific heat)
• Morphology (particle size distribution, sphericity)
• Chemical
• Proximate analysis
• Elemental (CHNS via combustion /chromatography; ICP-OES/MS)
• Bomb calorimetry
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How to characterize biomass
Proximate analysis
= The distinction into ash, water, volatile matter and fixed carbon – purely based on weight loss after oven-
treatment
1. Water content (MC, moisture content)
• Important property in thermochemical conversion: al water has to evaporated (except in
hydrothermal processes) and requires latent heat !
• Distinction in freely available and bound water
• Freely available water = water available above equilibrium concentration (e.g. pine: 9 wt.%
equilibrium concentration water at Tair = 20°C and RHair = 0.5)
• Bound water = total moisture content – free water. Is the water bound to the cell wall constituents.
• Moisture content determined gravimetrically, by drying up to 105°C
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How to characterize biomass
2. ash content
• The inorganic remnant after combustion
• Woody biomass < 1 wt.%; SRWC < 2 wt.%; energy grasses ~ 7 to 10 wt.%. Residues like digestate, sewage
sludge > 10 wt.%
• K, Ca (and other alkali and alkaline earth metals): can act catalytically at higher temperatures.
• K (+Si): ‘Low’ melting point, results in ash melting and ‘fouling’
• Cl, S: Yield HCl and H2SO4 and are corrosive.
3. Volatile matter (VM)
• The organic part that volatilizes at temperatures of 950 °C (volatilization does not require O2 !).
4. Fixed carbon (fC)
• The organic part that does not volatilize at a temperature of 950°C, or fC = 100 – Ash – MC – VM.
• Char = high fC content. The majority of the volatiles have been driven off in the pyrolysis.
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How to characterize biomass
Elemental composition (ultimate analysis)
• Ultimate analysis is the determination of weight fractions of H, S, O, N, C in the organic matter fraction in
the biomass (+ additionally metals, trace elements)
• C & H contribute to the HHV
• S undesired: results in SOx emissions during combustion. Biomass: typically < 0.1 wt.% however fossil fuels
> 1 wt.% ! Combustion of fossil fuels requires active removal of SOx from the flue gases by e.g. injection of
lime which happens in coal combustion (Ca(OH)2 which reacts with SOx to CaSO4).
• N undesired: yields NOx emissions during combustion. Usually, N content is low in fossil fuels but can be
high (> 1 wt.%) in certain biomass feedstocks due to the presence of proteins. Requires active removal
from flue gases by selective catalytic or non-catalytic reduction using NH3 as reductans.
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How to characterize biomass
Elemental composition (ultimate analysis)
• The atomic ratios of O/C en H/C determine the fuel
quality of biomass, biomass-derived products and
fossil fuels
• If O/C ↑ then HHV ↓. If H/C ↑ then HHV ↑.
• O/C and H/C ratios on the so-called Van Krevelen
diagram
Dehydratatie
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How to characterize biomass
Heating value
• Is the heat released during combustion (reaction with oxygen) per mass
unit of fuel/biomass.
• Measured by means of a bomb calorimeter
• Distinction between higher heating value (HHV) and lower heating
value (LHV)
• HHV = LHV + latent heat (heat of evaporation at 25°C) of water vapor in
the combustion gases
• At room temperature: latent heat of water is 2.5 MJ/kg
• HHV can be estimated based on the known elemental composition
(C,H,N,O,S and ash in wt.%) of the biomass (the so-called Dulong
formula),
Warmte
Roerder OntstekingThermometer
Thermisch geïsoleerd vat
O2-gas
Bom
Staalhouder met biomassa-monster
Water
Stirrer Ignition
Thermally insulated water vessel
Bomb
Sample holder
Heat
ashNOSHCkgkJHHV 1.211.154.1035.1003.11781.349/
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How to characterize biomass
Basic techniques to characterize biomass prior to thermochemical conversion:
• Physical
• Density (true, apparent and bulk)
• Thermal (conductivity and specific heat)
• Morphology (particle size distribution, sphericity)
• Chemical
• Proximate analysis
• Elemental (CHNS via combustion /chromatography; ICP-OES/MS)
• Bomb calorimetry
Gives substantial compositional information, but doesn’t tell quite as much as to what to expect in pyrolysis
• “Advanced” methods which are based on
thermal decomposition
• Thermogravimetry (TGA, TGA-MS, TGA-FTIR)
• Analytical pyrolysis
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How to characterize biomass
Thermogravimetric analysis
• Measuring weight loss evolution when heating a sample
• Low heating rates (i.e. 5, 10 °C/min)
• In inert (He, N2) atmosphere pyrolysis, or oxidative (O2)
atmosphere combustion
• Can be used to predict optimum pyrolysis temperature ranges
and expected char yields
• Can be used to perform proximate analysis
• TGA-MS, TG-FTIR: combined TGA and evolved gas analysis
through mass spectrometry or FTIR
Source: Setaram
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How to characterize biomass
Thermogravimetric analysis
Cellulose
HemicelluloseLignine
Ge
wic
ht
(w%
)
Sn
elh
eid
ge
wic
hts
ve
rlie
s
(w%
/°C
)
Temperatuur (°C)
0 200 400 600 800
0
20
40
60
80
1003.0
2.5
2.0
1.5
1.0
0.5
0.0
Mass (
%)
Mass loss r
ate
(%
per
°C)
Temperature (°C)
• E.g. lignocellulosic biomass
constituents
• TGA-dTG
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Analytical pyrolysis
• Originated out of the idea to
analyze solid (non-volatile) samples
in gas chromatography
• Uses pyrolysis to thermally
crack/decompose the sample into
volatile, GC-detectable compounds
• Decomposition pattern is unique to
the composition of the sample
tested
GCGCMSMS
Carrier gasPyrolysis oven
Chromatographic
column
Mass detector
Biomass sample
Sample cup
time
Working principle
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Analytical pyrolysis
Pyrolysis methods
1. Microfurnace
• Preheated tubular furnace (quartz or steanless
steel lined), flushed with carrier gas
• Small volume of furnace to reduce dead volume
• Sample introduction by dropping sample into the
furnace (by gravity), held in a open-ended holder
(cup)
• Liquid samples can be injected directly
• Sample (+holder) heating by convection
• Thermal inertia of the furnace (in EGA, evolved
gas analysis or gradient pyrolysis slow heating)
• E.g. Frontier lab
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Analytical pyrolysis
Pyrolysis methods
2. Filament pyrolyzer
• Sample placed in a quartz tube held within a Pt coil (on
a probe tip) or directly coated on a Pt strip (liquids)
• Coil is heated by electrical current (resistive heating) –
temperature is not measured but assumed from
resistivity and current
• The probe tip is place in a preheated cavity (
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Analytical pyrolysis
Pyrolysis methods
3. Curie point pyrolyzer
• Curie point temperature: the temperature at which a
ferromagnetic metal loses its magnetic properties
• Metal alloys are heated inductively to exactly the Curie
point
• The sample is in direct contact with the metal alloy (be
aware of potential catalytic effect)
• Only some select temperatures available, each alloy has
different Curie point temperature
• Can only heat rapidly to the Curie point temperature
• Sample size is limited
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Analytical pyrolysis
Pyrolysis methods
3. Curie point pyrolyzer
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Analytical pyrolysis
Pyrolysis methods
4. Laser pyrolyzer
• Heating of sample by means of laser light
• Fast heating, but exact pyrolysis temperature is not
known, also not exactly known how much sample is
pyrolyzed problems in reproducibility
• Only suited for materials that are opaque (i.e. light
absorbed), not for transparent materials (or absorbents
need to be added…)
• The beam can be directed to specific parts in the
biomass (i.e. different parts in plant tissues)
• Not common, not commercially available
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Analytical pyrolysis
Important parameters in analytical pyrolysis
• Heating rate (i.e. rate of heating before reaching pyrolysis temperature,
HRp):
• too low heating rate slow release of volatiles peak
broadening/poor separation on the GC
• Extra focus step may be required if HRp is low
• Too low heating rate secondary vapor-phase pyrolysis reactions
• Pyrolysis time (Dtp)
• Pyrolysis temperature (Tp,f)
• Sample size
• Too large sample’s heat transfer limitations
• Too small Low accuracy, catalytic wall effects may be dominant
• Interface temperature (Ti,f)
• Sufficiently high to avoid condensation of pyrolysis vapors
Run time
Te
mp
era
ture
T0
HRi
Ti,f
Tp,f
HRp
T0Dti
Dtp
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Analytical pyrolysis
Important parameters in analytical pyrolysis
• Extra focus step
• Directly on the column (cryofocus)
• Separate trap (tenax), trough gas
switching valve (also allows to have a
different carrier gas than the one in
which was being pyrolyzed)GCGCMSMS
Carrier gas
Pyrolysis oven
Chromatographic
column
Mass detector
Biomass sample
Sample cup
Cold or liquid N2
Pyrolysis oven
Cooling N2
GC column
Cold spot (trapping in
column)
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Analytical pyrolysis
Important parameters in analytical pyrolysis
• Extra focus step
• Directly on the column (cryofocus)
• Separate trap (tenax), trough gas switching
valve (also allows to have a different carrier
gas than the one in which was being
pyrolyzed)
Valve oven
Interface
Pt-filament coil
Quartz tube
Sample
Quartz wool
GC carrier gas
(helium)
Heated transfer line
GC/MS or
GC/FIDRotary
valve
Trap
Outlet
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Analytical pyrolysis: biomass
What to expect ?
Example: ~300 µg, sugarcane bagasse, 500°C, microfurnace pyrolyzer
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Analytical pyrolysis: biomass
Cellulose
Zheng et al., 2016
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Analytical pyrolysis: biomass
Hemicellulose
Patwardhan et al., 2011
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Analytical pyrolysis: biomass
Lignin
Kosa et al., 2011
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Analytical pyrolysis: case examples
1. Pyrolysis of mutant Poplar with deficient lignin biosynthesis
• Vercruysse et al. (2016)
• Lignin biosynthesis pathway
• The HCT-enzyme (hydroxycinnamoyl-CoA:shikimate
hydroxycinnamoyl transferase 1) forms the
branching point between H and G/S bioynthesis
• HCT-low or deficient mutants: accumulation of H-
units in lignin, also shown to reduce MW of lignin
polymers
• Hypothesis: in fast pyrolysis do these HCT-low
mutants,
• produce more monophenolic species ?
• and/or different spectrum of phenolic species ?
NH2
OHO
phenylalanine
OHO
cinnamic acid
OHO
OH
p-coumaric acid
O
OH
SCoA
p-coumaroyl-CoA
O
OH
SR
p-coumaroyl
shikimic/quinic
acid
O
OH
SR
OH
caffeoyl-shikimic/
quinic acid
O
OH
S
OH
CoA
caffeoyl-CoA
O
OH
SCoA
OMe
feruloyl-CoA
p-coumarylaldehyde
p-coumarylalcohol
O
OH
H
OH
OH
O
OH
OMe
HO
OH
OMe
H
HO
O
OH
OMe
H
MeO
OH
OMeMeO
OH
OH
OMe
OH
sinapaldehyde 5-hydroxy
coniferaldehyde
coniferaldehyde
sinapyl alcohol coniferyl alcohol
C4H PAL
HCT C3H HCT CCoA-OMT
COMT F5H
4CL
CCRCCR
CAD CAD CAD
H-lignin S-lignin G-lignin
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Analytical pyrolysis: case examples
1. Pyrolysis of mutant Poplar with deficient lignin biosynthesis
• Py-GC/MS, 500°C of homozygous (D73/D73), heterozygous (D73/+) mutants and wild type (+/+) poplar
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Analytical pyrolysis: case examples
1. Pyrolysis of mutant Poplar with deficient lignin biosynthesis
• Py-GC/MS, 500°C of homozygous (D73/D73), heterozygous (D73/+) mutants and wild type (+/+) poplar
• Detail of phenolics (according to degree of methoxylation) in py-GC/MS:
% H % G % S
WT 0.39 ± 0.09 35.23 ± 3.17 63.20 ± 3.18
Heterozygous 0.56 ± 0.10 35.03 ± 2.86 64.41 ± 2.93
Homozygous 6.98 ± 0.95 26.98 ± 0.42 66.04 ± 1.37
Compositional data from NMR:
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Analytical pyrolysis: case examples
2. Testing catalysts in catalytic pyrolysis
• Analytical pyrolysis has become more than just an
‘analytical’ technique, it also allows to perform
‘microscale’ pyrolysis experiments
• For instance, catalysts can be tested with respect
to activity/selectivity both ‘in-situ’ or ‘ex-situ’
(with secondary reactor – independently
temperature-controlled)
• Also, reactive gases may be supplied to test
hydropyrolysis or catalytic hydropyrolysis
• Case example: catalyst testing in CFP (Yildiz et al.,
2016)
GCGCMSMS
Carrier gas
Pyrolysis oven
Ex-situ catalytic reactor
Catalytic reactor oven
Reactant gas
Chromatographic
column
Mass detector
Biomass sample
Sample cup
time
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Analytical pyrolysis: case examples
2. Testing catalysts in catalytic pyrolysis
• In-situ testing (500°C)
• Feedstock: pine (300 µg)
• HZSM-5 (A) and metal-modified HZSM-5
based catalysts (A-M) in low (L) and high (H)
loading
• Pyrogram peak areas cumulated according
to chemical functionality
• Clearly, the aromatization activity of the
zeolite catalyst is seen
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Analytical pyrolysis: case examples
2. Testing catalysts in catalytic pyrolysis
• In-situ testing (500°C)
• Feedstock: pine (300 µg)
• HZSM-5 (A) and metal-modified HZSM-5
based catalysts (A-M) in low (L) and high (H)
loading
• Pyrogram peak areas cumulated according
to chemical functionality
• Clearly, the aromatization activity of the
zeolite catalyst is seen
• Comparison with a bench-scale reactor and
its subsequent pyrolysis liquid analysis
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Analytical pyrolysis: pitfalls, how to avoid them
• Difficult result comparison between different types of instruments, different protocols, etc…
• Most results are reported in relative abundance (good for analytical purposes but not if you want to
use the technique for ‘microscale’ pyrolysis) calibrate & report in absolute quantities/yields
• Be aware you’re only quantifying a certain part of the pyrolysis products (i.e. GC-detectable
volatiles)
• Statistical data processing is required
• Py-GC/MS results can not be 100% scaled to larger scale pyrolysis
• No vapor condensation
• Differences in heating rate/vapor residence time/feedstock particle morphology/…
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This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 764089
Prof. dr. ir. Frederik Ronsse
Contact:
Dept. of Green Chemistry and Technology
Ghent University
Coupure Links 653
B-9000 Ghent
Belgium